Thanoseid has all of the powers and ambitions of both villains, and is thus a very dangerous threat. He is the ruler of Apokolips, and his goal is to annihilate all of the universe save Apokolips and to win the favor of Lady Death by finding the Ultimate Anti-Life Nulifier. He teamed up with Doctor Doomsday during the events of the Secret Crisis to destroy the universe.
This combination of characters has a particular twist; Thanos' creator Jim Starlin has always admitted that Thanos was very directly inspired by Jack Kirby's Darkseid. During the DC/Marvel series that led to Amalgam, Darkseid called Thanos a pale imitation of himself. In the Avengers/JLA Crossover series, Darkseid found and briefly wielded the Infinity Gauntlet, once sought after by Thanos but discarded it as it had no power or use in his universe. Avengers' archer Hawkeye took one look at Darkseid and declared him worse than Thanos, once more acknowledging the characters' connection.